Chemical detection is a pressing necessity for many applications. Most chemical sensors are small area sensors, i.e., sensors with responsive regions having small lateral dimensions. These sensors are based on changes in characteristics such as electrochemical and optical properties. For example, as discussed in Peterson et al. U.S. Pat. No. 4,200,110 issued Apr. 29, 1980, a small area chemical sensor using fiber optics is made by enclosing a material that reacts with the chemical to be sensed and attaching this package to the end of an optical fiber. The material in the enclosure is chosen so that upon interaction with the chemical to be sensed it changes its optical properties. Thus, upon reaction, guided light entering the package interacts with the sensitive material and the presence of the chemical is detectable as a change in optical absorbance or fluorescence at one or more wavelengths. This change is observed in the light guided back along the fiber.
Some important applications require rapid response, i.e., response in peroids shorter than 1 minute from time of chemical release. For example, it is often desirable to continuously monitor for the presence of undesirable gases to ensure worker safety. Alternatively, it is often necessary to monitor for gases which indicate a malfunctioning chemical process. Small area sensors are limited by the transit time from the source of gas to the responding area of the sensor. Even the placement of a reasonable number of small area sensors is generally not entirely satisfactory. Despite the severe response limitations of small area sensors in distributed region applications, they have nevertheless been utilized for lack of a better alternative.
Attempts to expand the response region of sensors have led to undesirable results. In one approach (U.S. Pat. No. 4,560,248 issued Dec. 24, 1985) a relatively short region of an optical fiber core having the solid cladding removed was made porous. This porous area was then filled with a dye whose optical properties changed upon interaction with the chemical to be detected. However, the bare core of the sensor is susceptible to external damage, e.g., scratches, resulting in breakage. Additionally, the desired signal is rapidly attenuated by scattering at the porous glass/core interface. Thus, such a sensor is not robust and the production of a porous core over an extended region severly complicates fabrication and detection.
As described by Giuliani in U.S. Pat. No. 4,513,087 issued Apr. 23, 1985, a second approach to fabricating a distributed region sensor involves a capillary tube coated with a comparison that changes optical absorption properties upon interaction with the chemical to be detected. This change, in turn, induces a detectable wavelength dependent change in the intensity of the light guided by the annulus of the capillary tube. A capillary tube rather than a solid rod was used, according to Giuliani, to ensure sufficient interaction between the chemically sensitive composition and the guided light. Clearly, however, this configuration is quite susceptible to breakage, is susceptible to contamination, and is not easily configured along a curvilinear path. Thus, a flexible, robust distributed chemical sensor that lends itself to expedient fabrication has not been reported.